The present invention relates generally to a secondary electron detector and a charged particle beam apparatus. More specifically, the present invention relates to a secondary electron detector having a scintillator, which absorbs electrons and emits light, and a photomultiplier, which amplifies the light emitted from the scintillator, and a charged particle beam apparatus having the secondary electron detector.
The charged particle beam apparatus, such as a scanning electron microscope (SEM), an electron probe micro-analyzer (EPMA), a focused ion beam apparatus (FIB), or an electron beam thin film transistor (TFT) array inspection apparatus, irradiates a sample with primary charged particles and allows a secondary electron detector to detect secondary electrons emitted from the surface of the sample. The scanning electron microscope, which is the above-described charged particle beam apparatus, uses an electron gun to produce a beam of electrons as a primary electron beam, allows an objective lens or other converging unit to converge the electron beam on the sample, causes a scanning unit to perform a two-dimensional scan for the purpose of irradiating the sample with the electron beam, allows the secondary electron detector to detect secondary electrons generated upon electron beam irradiation, and obtains a scanned image of the sample by supplying a detection signal to a display unit that is synchronized with the electron beam scan.
FIG. 3 illustrates a publicly known secondary electron detector as an example of the above-described secondary electron detector. FIG. 3 is a cross-sectional view. The secondary electron detector 40 illustrated in FIG. 3 includes a scintillator 41 that emits light upon receipt of secondary electrons from a sample, a light guide 42 that guides the light emitted from the scintillator 41, a scintillator cap 43 that is disposed around the scintillator 41 to receive the application of a high voltage (e.g., 10 kV) for the purpose of enabling the scintillator to attract the secondary electrons, a photomultiplier (PMT) 44 that amplifies the light generated by the scintillator, and a socket 45 that accepts the photomultiplier 44. Reference should be made to JP 2004-14251-A2 and JP 2009-043594-A2.
The secondary electron detector 40 is structured so that the light guide 42 is mounted on a mounting member 47. The mounting member is mounted in a sample chamber, which is a vacuum chamber. This assures that the scintillator 41 is placed in a vacuum while the socket 45 is placed outside the sample chamber and under atmospheric pressure. Hence, a vacuum seal is formed between the light guide 42 and the mounting member 47. The light guide 42 is formed, for instance, by glass. The vacuum seal is formed by filling an adhesive 46 into a gap between the mounting member 47 and the circumference of the light guide 42. In FIG. 3, the reference numerals 46a and 46b denote a fill of the adhesive 46, and the reference numeral 48 denotes a cable that applies a high voltage to the scintillator cap 43. It should be noted that the vacuum seal may be formed with an O-ring.
In the secondary electron detector 40 illustrated in FIG. 3, however, weak light from the scintillator 41 passes through the light guide 42 and enters the photomultiplier 44. Therefore, the light is attenuated depending on a material of the light guide 42 and by its reflection from both end faces of the light guide 42. Consequently, the light from the scintillator 41 cannot be efficiently guided to the photomultiplier 44.
Further, in recent years, a scanning electron microscope in which an optical microscope and an electron microscope are disposed in the same sample chamber to observe the same sample has been made available. When such a scanning electron microscope is used, illumination light of the optical microscope enters from the scintillator 41 side. This illumination light can be blocked when a wavelength filter for blocking light within the wavelength region of the illumination light is disposed on the incidence side of the scintillator 41. However, light incident, for instance, from a lateral side of the light guide 42 enters the photomultiplier 44 and is detected. Therefore, the above scanning electron microscope is at a disadvantage in that an optical microscope image and an electron microscope image cannot be simultaneously observed.
In addition, a high voltage of several thousand volts is applied to the scintillator cap 43 while a voltage of several hundred volts is applied to the photomultiplier 44 so that electrons emitted from a photocathode are transferred to a dynode electrode. The applied voltage generates a potential on the surface of the photomultiplier 44. If the light guide is removed for direct contact for increasing detection efficiency, an electric discharge occurs between the above two members due to a potential difference therebetween. The electric discharge gives rise to a noise source, making it impossible to obtain an excellent image.
The present invention has been made in view of the above circumstances and provides a secondary electron detector and a charged particle beam apparatus that have a simple configuration and are capable of acquiring an excellent image with high detection efficiency while preventing an electric discharge.